1. Field of the Invention
This invention relates to copolymers, and more specifically relates to melt processable polyurethaneureas and to a method for their preparation.
2. Background of the Invention
Polyurethane block copolymers possess an outstanding balance of physical and mechanical properties and superior blood compatibility compared to other polymers such as silicone rubber, polyethylene, polyvinyl chloride and perfluorinated polymers. As a result, they have come to the fore as the preferred polymeric biomaterials for fabrication of various medical device components. Some important device applications for polyurethanes include peripheral and central venous catheters, coatings for heart pacemaker leads and the Jarvik heart.
Polyurethanes are synthesized from three basic components, a polyisocyanate, a polyglycol and an extender, usually a low molecular weight diol, diamine, aminoalcohol or water. If the extender is a diol, the polyurethane consists entirely of urethane linkages. If the extender is water, aminoalcohol or a diamine, both urethane and urea linkages are present and the polyurethane is more accurately and conventionally termed a polyurethaneurea. In this disclosure, polyurethaneurea will hereinafter be abbreviated as PUU.
The usual polyglycols are polyethylene glycol (PEG) and polytetramethylene ether glycol (PTMEG). Polypropylene ether glycol (PPG), while providing a polyurethane of desirable high softness, is infrequently used for polyurethanes intended for medical use because PPG requires a catalyst for reaction with isocyanates. The usual catalysts for polyurethane synthesis, such as dibutyl tin dilaurate, are toxic and contraindicated for medical grade polyurethane synthesis because of the danger of leaching into a patient's body fluid.
Polyurethanes and PUU develop microdomains conventionally termed hard segments and soft segments, and as a result are often referred to as segmented polyurethanes. The hard segments form by localization of the portions of the polymer molecules which include the isocyanate and extender components and are generally of high crystallinity. The soft segments form from the polyether glycol portions of the polymer chains and generally are either noncrystalline or of low crystallinity. Crystallinity and hard segment content are factors which contribute to melt processability.
It is known that PEG is clear viscous liquid at molecular weights below about 900 and is an opaque white solid of increasing hardness as the molecular weight increases above 900. PPG is essentially noncrystalline regardless of its molecular weight whereas PTMEG develops some crystallinity at higher molecular weight. With PTMEG, the normal chain mobility of the soft segment is decreased as the level of crystallinity increases due to the infusion of crystallites of the soft segment into the hard segment. This in turn affects the elastomeric character of the polymer. Nevertheless, polyurethanes made from PTMEG are generally melt processable, but catheters extruded therefrom are less flexible than catheters fabricated from PEG and PPG.
PPG, being totally noncrystalline, gives a polyurethane having maximum phase separation between the hard and soft segments. As a result, PPG derived polyurethanes are soft and elastomeric, and the softness is affected by small changes in temperature. Thus, at body temperature, a typical PPG polymer is about 75 to 90% softer than at room temperature as compared to a 60 to 75% change shown by a typical PTMEG derived polyurethane.
As is well-known in the art, PUU made with diamine extenders are generally not melt processable regardless of the polyglycol used as the soft segment. For example, a PUU well-known as an industrial fiber (Lycra.RTM. DuPont de Nemours and Co.) has been extensively studied under the trade name Biomer.RTM. (Ethicon Corp.) for fabrication of various biomedical devices. A review of these studies and the many salubrious properties of PUU has been presented by Phillips et al., The Use of Segmented Polyurethane In Ventricular Assist Devices and Artificial Hearts, in Synthetic Biomedical Polymers, M. Szycher and W. J. Robinson, ed. Technomic Publishing Co., Inc., Westport, Conn., 1980, page 39. However, as stated by Phillips et al., Biomer.RTM. presents some fabrication difficulties that limit production techniques. Biomer.RTM. has a melt temperature higher than the decomposition temperature of the urethane functionality and therefore can be spun or cast only from solution, i.e., it cannot be melt extruded or injection molded. Severe limitations are thereby imposed on its fabrication latitude. Further, it is essentially insoluble in all solvents except DMAC which of course must be completely removed if the product is to be used in a biomedical article.
Since PUUs, such as Biomer.RTM. are well known to be highly biocompatible, a PUU which would combine the biocompatibility of Biomer.RTM. with the melt processability of polyurethanes would be a desirable product. One approach to this objective is disclosed in copending application serial number 345,800, filed on May 1989, of common assignee with the present invention. This application discloses a melt processable PUU having both a diol and diamine chain extender. The instant application discloses another class of melt processable PUU.